"Add user authentication" is sort of "build a webapp 101" in most backend framework's (Django, Rails, Laravel etc), except there aren't that many resources around that for Node since the most widely adopted frameworks are pretty much just routers.
You could roll your own Auth with a couple of tables (user table, sessions table), using bcrypt to hash/salt the passwords (unnecessary if you only do social logins), I personally use sequelize as an ORM but that's not required.
Or as Stephen said, use passport.js, haven't used it personally but it seems like the goto in the Node community.
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